The present invention relates to a cooling system for an internal combustion engine.
An internal combustion engine generally incorporates a cooling system which comprises water jackets formed in the engine housing including a cylinder block and a cylinder head, a radiator, a water pump, and conduit means which connect the water jackets, radiator, and water pump in a manner of establishing a cooling water flow circuit. In a conventional cooling system of this type, the cooling water delivered from the water pump is first introduced into a water jacket formed in the cylinder block through an inlet port of the water jacket provided at a front portion thereof and, after having flowed through the water jacket of the cylinder block, the cooling water is then introduced into a water jacket formed in the cylinder head through a connecting passage formed in a joining portion of the cylinder block and the cylinder head so that the cooling water then flows through the water jacket of the cylinder head. After having flowed through the water jacket of the cylinder head, the cooling water then flows out of the water jacket of the cylinder head through an outlet port formed at a front portion of the cylinder head and is led towards the radiator through a conduit. After having flowed through the radiator while radiating heat therefrom, the cooling water is returned to the intake port of the water pump through a conduit.
The temperature of the cooling water circulated through the cooling system for an internal combustion engine is controlled to be in a certain range so as not to cause any undesirable overheating of the engine on the one hand while not, on the other hand, causing an overcooling of the engine, which would deteriorate the combustibility of fuel in the cylinders. In accordance with these requirements, the temperature of cooling water is generally controlled to be within the range 80.degree.-90.degree. C.
In gasoline engines, if the target cooling water temperature is set at a relatively low value, octane number requirement, i.e., the minimum octane number of gasoline required for avoiding knocking in the engine, lowers, thereby enabling engine performance and/or economy of operation to be improved, while on the other hand when the target cooling water temperature is lowered, the disadvantage occurs that the emission of HC in exhaust gases increases.
Knocking in gasoline engines is caused by self-ignition of fuel-air mixture and is more apt to occur when fuel-air mixture is subjected to a higher temperature and higher pressure condition. Since a high temperature and high pressure condition such as to cause self-ignition of the fuel-air mixture occurs when the fuel-air mixture is compressed in the combustion chamber of an engine by the piston being shifted approximately to the top dead center, the possibility of knocking is not related to the temperature of the side wall of the cylinder but is substantially related to the temperature of the top wall of the combustion chamber which is provided by the cylinder head. In other words, the octane number requirement increases as the temperature of the top wall of the combustion chamber increases.
In view of the abovementioned two facts with regard to engine performance, it is desirable that the temperature of the cooling water for effecting the cooling of the top wall of the combustion chamber should be set at a relatively low value while the temperature of the cooling water for effecting the cooling of the side wall of the cylinder is set at a relatively high value, whereby it is to be expected that the octane number requirement is lowered while the emission of HC in exhaust gases is also lowered.
However, in a conventional cooling system having a single cooling water flow circuit which incorporates the water jackets formed in the cylinder block and the cylinder head in series, the cooling water is first passed through the water jacket of the cylinder block and is then passed through the jacket formed in the cylinder head so that the natural head difference caused in the circuit due to gradual heating up of cooling water is effectively utilized for helping to drive the cooling water through the circuit. In this case, therefore, the temperature of the cooling water flowing through the water jacket of the cylinder head is higher than that of the cooling water flowing through the water jacket of the cylinder block, contrary to the aforementioned requirement.